Inkjet head and method for manufacturing the same

ABSTRACT

A method for manufacturing an inkjet head includes providing a piezoelectric substrate having a porous structure, a diaphragm on the porous structure, and a piezoelectric substance layer on the diaphragm, and forming a cavity by etching out the porous structure from the piezoelectric substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an inkjet head that sprays drops of inkon a recording medium, such as paper, to form an ink image, and alsorelates to a method for manufacturing the inkjet head.

2. Description of the Related Art

The following are examples of conventional technologies of the inkjethead.

-   (1) Japanese Patent Publication No. 2976479

Conventionally, a pressure-generating means that sprays drops of inkfrom a cavity in the inkjet head has been provided to each cavity, forexample, by an adhesion process. However, according to Japanese PatentPublication No. 2976479, the pressure-generating means is provided on asilicon substrate by a process other than an adhesion process.

-   (2) Japanese Patent Laid-Open No. 07-276636

Japanese Patent Laid-Open No. 07-276636 defines the orientation of acrystal plane on a cavity wall in a method for forming the cavity byetching a silicon substrate.

Problem 1: In such conventional technologies, the shape of the cavitydepends on the anisotropic etching of the single-crystal silicon. Sincethe etching, in turn, depends on the crystal structure of thesingle-crystal silicon, the shape of the cavity is limited by thecrystal structure of the single-crystal silicon. In general, the cavityhas a (111) face of the single-crystal silicon. The etching rate is lowon the (111) face. Thus, the etching based on the orientation of thecrystal plane produces a cavity wall that is not perpendicular to thesilicon substrate, resulting in a lower cavity density.

The cavity wall is required to have an affinity for ink to prevent thedeposition of air bubbles.

Problem 2: Conventionally, the etching of the silicon substrate has beenperformed by selective etching based on the difference in theconcentration of doped p-type impurities. However, the selection ratioof the selective etching is several tens at the highest. Thus, when botha thin film portion of the substrate and the cavity portion are made ofsilicon, the thickness of the thin film portion may be poorly controlledand may vary. Japanese Patent Laid-Open No. 2002-234156 discloses amethod using a silicon-on-insulator (SOI) substrate, in which a buriedsilicon oxide layer serves as an etch stop material. In alkalineetching, the etching rate of silicon oxide is less than one-thousandthof that of silicon and accordingly the selectivity is excellent.

However, in alkaline etching, heat treatment during the formation of adiaphragm or the subsequent formation of pressure-generating means orperipheral circuitry may cause precipitation of oxygen in the substrate.The precipitated oxide acts as a mask during the etching because of itslow etching rate for an alkaline solution, and thus may cause nonuniformetching. Furthermore, such an oxide deposited on the diaphragm may causenonuniform mechanical properties in the diaphragm, leading to fractureor crack of the diaphragm.

Problem 3: An SOI wafer is about 4 to 10 times as expensive as asingle-crystal silicon wafer. In addition, when an SOI wafer having athick thin-film layer is manufactured by lamination and polishing,variations in the thickness of the SOI layer, which are about ±0.5 μm,cause variations in the thickness of the thin film portion.

SUMMARY OF THE INVENTION

To solve the problems described above, the present invention provides amethod for manufacturing an inkjet head, comprising providing a firstsubstrate that includes a piezoelectric substance layer and a diaphragmformed on a porous structure, and etching out the porous structure fromthe first substrate to form a cavity.

The present invention also provides an inkjet head comprising apiezoelectric substance layer, a diaphragm provided with thepiezoelectric substance layer, and a cavity, wherein the diaphragm ismade of silicon containing 5×10¹⁷/cm³ or less of oxygen.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 is a perspective view of an inkjet head according to anembodiment of the present invention.

FIG. 2 is a transverse cross-sectional view of a portion of the inkjethead of FIG. 1 showing piezoelectric film in greater detail.

FIGS. 3A to 3F are schematic views illustrating a method formanufacturing an inkjet head according to an embodiment of the presentinvention.

FIGS. 4A to 4C are schematic views illustrating a process formanufacturing a nozzle plate according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The best mode for carrying out the invention will be described in detailwith reference to the drawings. FIG. 1 shows the structure of an inkjethead according to an embodiment of the present invention. The inkjethead includes a discharge opening 1, a communicating hole (liquid path)2 that connects the discharge opening 1 with a cavity 12, a commonliquid chamber 4, a diaphragm 5, a lower electrode 6, a piezoelectricfilm (piezoelectric substance layer) 7, and an upper electrode 8. Thepiezoelectric film 7 is rectangular in FIG. 1 but may be elliptical,circular, or parallelogrammatic.

The piezoelectric film 7 will be described in detail with reference toFIG. 2. FIG. 2 is a transverse cross-sectional view of a portion of theinkjet head of FIG. 1 showing the piezoelectric film in greater detail.The piezoelectric film 7 is composed of a first piezoelectric substancesublayer 9 and a second piezoelectric substance sublayer 10. Thediaphragm 5 and the lower electrode 6 may be separated by a buffer layerthat controls crystallinity. The lower electrode 6 and the upperelectrode 8 may have a multilayer structure. The cross section of thepiezoelectric film 7 is rectangular in FIG. 2 but may be trapezoidal orinverted trapezoidal. The first piezoelectric substance sublayer 9 andthe second piezoelectric substance sublayer 10 may be exchanged witheach other, depending on the method of fabricating the device. Even whenthe first piezoelectric substance sublayer 9 and the secondpiezoelectric substance sublayer 10 are exchanged with each other, thepresent invention can have the same effect.

The lower electrode 6 extends longer than the piezoelectric film 7. Theupper electrode 8 extends in the direction opposite to the lowerelectrode 6 and is connected to a power supply (not shown). In FIGS. 1and 2, the patterned lower electrode 6 may be formed independently ofthe piezoelectric film 7.

The thickness of the diaphragm 5 in the inkjet head according to thepresent invention is in the range of 0.1 to 50 μm, and can be in therange of 0.5 to 10 μm, or in the range of 1.0 to 6.0 μm. When a bufferlayer is disposed between the diaphragm 5 and the lower electrode 6, thetotal thickness of the diaphragm 5 and the buffer layer is in the rangedescribed above. The thicknesses of the lower electrode 6 and the upperelectrode 8 are in the range of 0.05 to 0.4 μm and can be in the rangeof 0.08 to 0.2 μm. The width of a cavity 12 in a silicon substrate 11 isin the range of 30 to 180 μm. The length of the cavity 12 depends on thenumber of drops of ink to be sprayed and is generally in the range of0.3 to 6.0 mm. The discharge opening 1 may be circular or star-shapedand can have a diameter of 7 to 30 μm.

The discharge opening 1 can taper down to a narrow tip. The length ofthe communicating hole 2 can be in the range of 0.05 to 0.5 mm. When thecommunicating hole 2 has a length greater than 0.5 mm, the dischargespeed of the drops of ink may be decreased. On the other hand, when thecommunicating hole 2 has a length smaller than 0.05 mm, the dischargespeed of the drops of ink from each discharge opening may vary greatly.

The lower electrode 6 and the upper electrode 8 may be made of ametallic material or an oxide material. Examples of the metallicmaterial include Au, Pt, Ni, Cr, and Ir. The metallic material may belaminated on Ti or Pb. Examples of the oxide material include astrontium titanium oxide (STO), a strontium ruthenium oxide (SRO), IrO₂,RuO₂, and Pb₂Ir₂O₇, each doped with La or Nb. Desirably, the lowerelectrode 6 and/or the upper electrode 8 has a crystal structure of themetallic material or the oxide material. The lower electrode 6 and theupper electrode 8 may or may not be made of the same material and may ormay not have the same structure. One of the lower electrode 6 and theupper electrode 8 acts as a common electrode and the other acts as adrive electrode.

A method for manufacturing the inkjet head according to the presentinvention will be described below with reference to FIG. 3. In themethod for manufacturing the inkjet head according to the presentinvention, the production of a piezoelectric substrate Al mainlyinvolves a process for producing a nozzle pattern on the backside of thesubstrate, an anodization process, a process for forming a diaphragm, aprocess for forming a piezoelectric substance, and an etching process.Then, the piezoelectric substrate A1 is laminated to a nozzle plate A2in a lamination process.

In FIGS. 3A to 3F, the piezoelectric substrate A1 includes cavities 30and a diaphragm 18. The nozzle plate A2 includes a communicating hole, adischarge opening, and a common liquid chamber.

1. Formation of Cavity (1) Patterning of Anodization Area

As shown in FIG. 3A, a film resistant to anodization 16 is formed on aprincipal surface of a single-crystal silicon substrate 15 that has topand bottom polished surfaces and has a thickness of 625 μm, except onsurface areas where porous silicon layers 17 are to be formed.

The anodization-resistant film 16 may be formed by any method and may beformed by a patterning technique that is widely used in thesemiconductor process. The material and the thickness of theanodization-resistant film 16 are determined such that theanodization-resistant film 16 is not detached and does not dissolveduring the formation of the porous silicon layers 17. For example, theanodization-resistant film 16 is made of silicon nitride, silicon oxide,a resist, a resin (acryl resin or epoxy resin), or wax (for example,Apiezon Wax (trade name) or Electron Wax (trade name)). Alternatively,the areas where the porous silicon layers 17 are to be formed may be ofa p-type or a p+-type, and the area where the porous silicon layers 17are not to be formed may be of a p⁻-type or an n-type.

(2) Formation of Porous Silicon Layer (FIG. 3A)

The porous silicon layers 17 may be formed by the anodization of thesingle-crystal silicon substrate 15. In the anodization, an electriccurrent is applied to the substrate in an aqueous solution containinghydrofluoric acid. The principal surface of the single-crystal siliconsubstrate 15 serves as a cathode.

The anodization proceeds only in the area where theanodization-resistant film 16 is not formed. The thickness of the poroussilicon layers 17 is controlled, for example, by the duration of theanodization. The thickness of the porous silicon layers 17 is determinedin view of the fact that the porous silicon layers 17 are eventually tobe converted into the cavities 30. The porous silicon layers 17 may beformed from the top surface to the bottom surface of the single-crystalsilicon substrate 15.

(3) Formation of Diaphragm (FIG. 3B)

The anodization-resistant film 16 is removed. Then, a non-porous singlecrystal diaphragm 18 is formed, for example, by thermal CVD, plasma CVD,molecular beam epitaxy (MBE), or liquid-phase epitaxy. The non-poroussingle crystal diaphragm 18 can be made of silicon.

When the anodization-resistant film 16 is made of single-crystalsilicon, it need not to be removed. The porous silicon layers 17 may beselectively oxidized before the formation of the non-porous singlecrystal diaphragm 18.

(4) Formation of Pressure-Generating Means (FIG. 3C)

A PZT piezoelectric substance layer 20 and accompanying electrode layers21 and 22 may be formed on the non-porous single crystal diaphragm 18formed on the porous silicon layer 17 in the following manner.

A common electrode layer 21, which is made of Pt, Cr and/or Ni and has athickness of 1 μm, the piezoelectric substance layer 20, which is madeof PZT and has a thickness of 10 μm, and an individual electrode layer22, which is made of Pt, Cr and/or Ni, are formed on the non-poroussingle crystal diaphragm 18 by sputtering or ion plating. Then, a resistpattern serving as a mask is formed on the individual electrode layer22. Then, ion etching or reactive ion etching of the common electrodelayer 21, the individual electrode layer 22, and the piezoelectricsubstance layer 20 produces a common electrode 21′, an individualelectrode 22′, and a piezoelectric substance 20′. At the same time, anoscillator and a wiring are formed.

(5) Removal of Porous Layer (FIGS. 3D and 3E)

The porous silicon layers 17 are removed from the backside of thesingle-crystal silicon substrate 15. If the porous silicon layers 17 areexposed at the backside after the formation of the porous silicon layers17, an exposure process will not be required. If the porous siliconlayers 17 are not exposed at the backside, the single-crystal siliconsubstrate 15 is lapped, ground, polished, or etched to expose the poroussilicon layers 17.

Then, the porous silicon layers 17 in the single-crystal siliconsubstrate 15 are etched, for example, with a solution containinghydrofluoric acid. A solution containing hydrofluoric acid is suitablefor an etchant, in particular when the porous silicon layers 17 havepreviously been oxidized. However, the etchant is not limited to asolution containing hydrofluoric acid. If an oxide is not found on theporous wall of the porous silicon layers 17 or has previously beenremoved from the porous wall, an aqueous alkaline solution may also beused as an etchant.

The etching produces the cavities 30, an ink feed channel, and a commonink channel. At the same time, a thin film portion 19 made of a siliconsingle crystal is formed.

In the process (5), the single-crystal silicon substrate 15 is reducedin thickness and therefore is liable to break. Thus, it is desirablethat, before the process (5), the single-crystal silicon substrate 15 befixed on a supporting substrate, for example, with a resin, such as anadhesive or wax, or a double-faced adhesive tape.

(6) Formation of Nozzle Plate (FIGS. 4A to 4C)

A manufacturing process and the structure of the nozzle plate A2 will bedescribed below with reference to FIGS. 4A to 4C. The nozzle plate A2may be made of any material that can form the nozzle. Examples of such amaterial include glass, a resin, and a single-crystal silicon substrate.A stable single-crystal silicon substrate that has the same coefficientof thermal expansion as that of the piezoelectric substrate A1 issuitable for the material. The nozzle may be formed in the followingmanner.

In FIGS. 4A to 4C, SiO₂ films 61 having a thickness of 0.1 μm are formedon the top surface and the bottom surface of a double-sided polishedsingle-crystal silicon substrate 60 having a thickness of 100 μm bythermal oxidation. Then, a resist layer 63 is formed over the entiresurface of the upper SiO₂ film 61. Another resist layer 63 is formed onthe lower SiO₂ film 61, except the areas 64 corresponding to theopenings of the cavities 30 in the piezoelectric substrate A1, so as tohave a crystal edge in the [110] direction (FIG. 4A).

The SiO₂ film 61 at the areas 64 is removed by etching and then theresist layers 63 are removed. Then, the single-crystal silicon substrate60 is anisotropically etched with a mixture of pyrocatechol,ethylenediamine, and water (FIG. 4B). Then, the SiO₂ films 61 areremoved. In this way, nozzles 70 having an outlet 71, which is smallerin diameter than the openings of the cavities 30, are formed (FIG. 4C).The positions of the nozzles 70 coincide with the positions of thecavities 30.

(7) Bonding of Piezoelectric Substrate A1 and Nozzle Plate A2 (FIG. 3F)

The piezoelectric substrate A1 is bonded to the nozzle plate A2 with thepiezoelectric substance layer 20 and the nozzle outlet 71 facingoutward. A voltage of 1000 V is applied between the negatively chargedpiezoelectric substrate A1 and the positively charged nozzle plate A2 at400° C. to bond them anodically.

In the present invention, at least part of the side wall of the cavity30 and the non-porous single crystal diaphragm (thin film portion) 18are made of a silicon single crystal in one piece. The sidewall of thecavity 30 is perpendicular to the non-porous single crystal diaphragm 18or tapers down to the nozzle outlet 71. The surface of the thin filmportion 18 in the cavity 30 has bumps and dips having a height of atleast 5 nm at intervals less than 50 nm (bumps and dips are also formedon the sidewall of the cavity 30 in FIG. 3E).

According to the present invention, the shape of the cavity isprincipally defined by the anodization from the diaphragm side. Theshape of the anodized portion is uniform along the electric line offorce. Thus, there are no hollows in the corners, unlike in the methodusing an SOI substrate. Furthermore, the sidewall of the cavity and thesurface of the thin film portion in the cavity have bumps and dips,which have been formed by the etching of the porous silicon layer andhave a height of at least 5 nm at intervals less than 50 nm. Thisimproves the wettability of these surfaces by ink.

The thin film portion is made of a single-crystal silicon containing5×10¹⁷/cm³ or less of oxygen. According to the present invention, thecavity is formed by selective etching of the porous silicon. Thus, forexample, a mixture of hydrofluoric acid and nitric acid or oxygenatedwater is used instead of an alkaline solution. When the concentration ofoxygen in the thin film portion is high, an oxygen precipitate is formedin the single-crystal silicon substrate by heat treatment. Thus, theoxygen precipitate in the thin film portion may be etched, causingdamage to the thin film portion. When the concentration of oxygen in thethin film portion is 5×10¹⁷/cm³ or less, however, oxygen precipitationhardly occurs as compared with a typical CZ substrate, and therefore thethin film portion is rarely etched during the removal of the poroussilicon layer. Widely commercialized silicon substrates made by thecrystal pulling method (CZ method) contain over 1×10¹⁸/cm³ of oxygen,and the heat treatment thereof causes oxygen precipitation. But oxygenprecipitation hardly occurs when the oxygen concentration is less than5×10¹⁷/cm³.

The concentration of oxygen in the single-crystal silicon of thesidewall of the cavity is 5×10¹⁷/cm³ or more. The sidewall is oftenformed by alkaline etching. A higher oxygen concentration causes oxygenprecipitation during the formation of a thin film, a PZT film, orperipheral circuitry. The oxygen precipitate is not etched duringalkaline etching and acts as a mask when a cavity or an ink feed channelis formed, causing a problem that a desired shape cannot be obtained bythe etching.

According to the present invention, the cavity is formed by selectiveetching of the porous silicon, for example, using a mixture ofhydrofluoric acid and nitric acid or oxygenated water, instead of analkaline solution. Thus, the problem described above does not occur.

Furthermore, the single-crystal silicon in the thin film portion is of ap-type or an n-type, and the single-crystal silicon constituting thesidewall of the cavity is of a p-type. The concentration of p-typecarriers in the sidewall of the cavity is higher than that in the thinfilm portion.

The single-crystal silicon constituting the sidewall of the cavity is ofa p⁺-type. In addition to the selective etching of the porous siliconlayer for the formation of the cavity, when etching is required to formthe ink feed channel or the like, the p⁺-type single-crystal silicon canbe predominantly etched over a p⁻-type or n-type single crystalepitaxial silicon of the thin film portion using a mixture ofhydrofluoric acid, nitric acid, and acetic acid (J. Electrochem. Soc.144 (1997) p. 2242). A 1:3:8 mixture of hydrofluoric acid, nitric acid,and acetic acid is recommended as an etchant. Such a hydrofluoricacid-based etchant can etch silicon oxide. Thus, the problem of theoxygen precipitate does not occur.

A hydrofluoric acid-based etchant cannot be used in selective etching ofthe conventional SOI wafer using silicon oxide as an etch stop layer.However, when an epitaxial silicon layer that is not of the p⁺-type isused as the etch stop layer, as in the present invention, a hydrofluoricacid-based etchant can be suitably used.

Furthermore, since the thin film portion is made of epitaxialsingle-crystal silicon, it is free from crystal-originated particles(COP), which can form a through-hole in a thin film having a thicknessless than 1 micron.

The method according to the present invention comprises a process forremoving the porous silicon to form the cavity. According to the methodof the present invention, unlike the conventional method, the shape ofthe cavity is principally defined by the anodization from the diaphragmside. The selection ratio of the selective etching of the porous siliconis at least 1000. Thus, the thin film portion maintains a uniformthickness during the removal of the porous silicon.

According to the present invention, the thickness of the single-crystalsilicon thin film portion constituting the diaphragm can be reduced toabout 0.1 to 50 μm. In addition, the diaphragm can be accurately formedin one piece since no adhesion process is required to be employed. Sincethe nozzle plate is made of the same material as the piezoelectricsubstrate, deformation due to a difference in the coefficient of thermalexpansion between the nozzle plate and the piezoelectric substrate doesnot occur during or after their bonding. This also ensures highdimensional accuracy of the inkjet head. The reduced thickness of theoscillator allows a small cavity to generate a sufficient displacementat low voltage. Thus, a small, highly integrated, reliable inkjet headoperable at low voltage can be provided at low cost. In addition, ashortened ink feed channel allows the inkjet head to remove air bubblesconsistently.

As many apparently widely different embodiments of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments thereof except as defined in the claims.

This application claims the benefit of Japanese Application No.2004-258367 filed Sep. 6, 2004, which is hereby incorporated byreference herein in its entirety.

1.-7. (canceled)
 8. An inkjet head comprising: a piezoelectric substancelayer; a diaphragm provided with the piezoelectric substance layer; anda cavity, wherein the diaphragm is made of silicon containing 5×10¹⁷/cm³or less of oxygen.
 9. The inkjet head according to claim 8, wherein asidewall of the cavity is made of silicon containing 5×10¹⁷/cm³ or moreof oxygen.
 10. The inkjet head according to claim 8, wherein the cavityis connected to a nozzle plate having a nozzle.